mwifiex: support STATION_INFO_SIGNAL_AVG

This patch adds the support for updating average signal information
in dump_station().

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>